The preparation of isocyanates, in particular diisocyanates, in the gas phase has been described in the prior art for a relatively long time and is used industrially, in particular for the production of toluylene diisocyanate, 1,6-hexamethylene diisocyanate, isophorone diisocyanate and diisocyanato-dicyclohexylmethane. In all processes there is formed a gaseous crude product which comprises at least isocyanate, hydrogen chloride and unreacted phosgene (phosgene is always used in excess) and which must be worked up further in order to obtain the desired isocyanate in pure form.
Such a process is described, for example, in EP 0 289 840 B1. The diisocyanates formed in a tubular reactor are not thermally stable at the reaction temperatures of up to 500° C. Rapid cooling of the reaction gases after the phosgenation reaction to temperatures below 150° C. is therefore necessary in order to avoid the formation of undesirable secondary products by the thermal decomposition of diisocyanate or by a further reaction. In EP 0 289 840 B1 or EP 0 749 958 B1, the gaseous mixture continuously leaving the reaction chamber, which comprises inter alia diisocyanate, phosgene and hydrogen chloride, is to that end passed into an inert solvent, for example dichlorobenzene. A disadvantage of this process is that the flow speed with which the gas mixture is passed through the solvent bath must be chosen to be relatively low because solvent and the compounds dissolved therein would be entrained if the speed were too high. The liquid compounds would have to be separated from the gas in a subsequent step. A further disadvantage is that, owing to the low flow speed and a low heat transfer, large solvent containers must be used in order to achieve cooling.
Also known are processes which use heat exchangers for cooling the reaction gases and/or relax the gases in vacuo (DE 101 58 160 A1). The disadvantage of heat exchangers is that, because of the poor heat transfer, large exchange surfaces, and thus large heat exchangers, are required for effective cooling. In addition, deposits of solids on the comparatively cold surfaces of the heat exchangers can occur owing to secondary reactions of the gas mixture, such as, for example, decomposition, polymerisation or precipitation.
In the process according to EP 1 761 483 B1, it is attempted to shorten the dwell time between the end of the reaction and the cooling zone by providing a region of reduced flow cross-section between the reaction zone and the zone in which termination of the reaction is effected.
Application WO2007/014936 A2, method for producing isocyanates (in the gas phase), describes a quenching zone in which the gaseous crude product is cooled rapidly by injection of a quenching liquid. In the quenching zone, the reaction mixture, which consists substantially of the isocyanates, phosgene and hydrogen chloride, is mixed intensively with the injected liquid. Mixing is carried out in such a manner that the temperature of the reaction mixture, starting from 200 to 570° C., is lowered to 100 to 200° C. and the isocyanate comprised in the reaction mixture is transferred wholly or partially into the injected liquid droplets by condensation, while the phosgene and the hydrogen chloride remain substantially wholly in the gas phase. Solvents, isocyanate mixtures and solvent/isocyanate mixtures are mentioned as possible quenching liquids. Mention is made of the injection of a quenching liquid to cool the reaction mixture and selectively dissolve the diisocyanate that has formed in the solvent, a first separation into a liquid phase and gas phase comprising predominantly phosgene and hydrogen chloride as constituents being carried out. The two phases are subsequently fed to a corresponding working up. Possible ways of optimising this method step are not discussed.
WO 2011/003532 A1 also discloses a method for rapidly cooling the gaseous reaction mixture by injecting a quenching liquid into the gas mixture flowing continuously from the reaction zone into the downstream quenching zone.
The injection of quenching liquid by means of at least two spray nozzles, which are arranged at the entry to the quenching zone, is disclosed in EP 1 403 248 B1. Suitable quenching liquids here are organic solvents or a mixture of different organic solvents which do not react with the diisocyanate that has formed. A solution of the diisocyanate that has formed in a suitable organic solvent can also be used, which reduces the amount of solvent used. The diameter of the quenching zone can be larger or smaller than the diameter of the reaction zone. Quenching of the reaction gases can take place in one stage or in a plurality of stages.
This system is optimised according to the teaching of EP 1 935 875 B1 in that, in order to terminate the reaction, the reaction mixture is guided from the reaction chamber through a cooling stretch into which liquids are injected in two zones, so that direct cooling in the cooling stretch takes place in one stage (i.e. yielding only one condensation mixture) in two or more cooling zones connected one behind the other. At least in the second zone, a cooling liquid is used that comprises the prepared isocyanate in considerable amounts (see patent claim 1, last paragraph). The diisocyanate produced is thereby obtained in a common condensation mixture. This mixture is preferably collected in a liquid collecting vessel arranged beneath the cooling stretch. This condensation mixture can be discharged in order to separate off the isocyanate that has been produced or, preferably after cooling has taken place, can partially be fed back to one or more cooling zones of the cooling stretch. A possible disadvantage of the use of the liquid crude product mixture from the gas-phase phosgenation is the occurrence of contamination of the described cooler before entry into the quencher. Causes of this can be undesirable secondary products or polymer compounds from the phosgenation reaction. In addition to the condensation mixture in the collecting vessel, there is obtained downstream of the cooling stretch a gas stream comprising at least hydrogen chloride, phosgene, optionally solvent, and the isocyanate that has been produced. This gas stream is removed from the collecting vessel and fed to a washing column, where it is largely freed of its isocyanate components. This washing preferably takes place counter-currently with solvent. The wash phase so obtained, consisting of diisocyanate and predominantly solvent, is used in a preferred embodiment as the quenching liquid of the first cooling zone of the cooling stretch. The residual gas from the washing column consists substantially of phosgene, hydrogen chloride and solvent. These vapours leave the column at the top, whereby, in a preferred embodiment, by means of partial condensation, the solvent component is largely retained by way of two condensers having different coolant temperatures and is fed back to the column as partial condensate. The residual gas obtained thereafter, which consists substantially of phosgene, hydrogen chloride and solvent residues, is then treated further in a manner known per se, as described, for example, in EP 1 849 767 B1.
The use of different suitable quenching liquid streams is likewise mentioned in EP 1 935 876 A1. Reference is also made thereby to the use as quenching liquid of the washing liquid from the gas washing of the vapours leaving the condensate collecting vessel after the quencher.
EP 2 196 455 A1 also refers to a plurality of cooling zones in the quenching stage. Mention is made here for the first time of the integrated combination of the cooling zones of a plurality of reactors with a quenching stage.
WO 2010/063665 A1 makes reference to a possible problem of the quenching variants known hitherto. If at least a portion of the quenching liquid is removed from the collecting vessel after the quencher, that is to say the liquid crude product, there is the possibility that solids may be present, which can block the quenching nozzles. Various techniques, such as, for example, centrifugation, removal by distillation of the liquid component provided for the quenching, or filtration, are described. In order to adjust the temperature of the chosen quenching stream for the problem posed, the stream can be cooled, or heated, by means of a heat exchanger. This specification discloses (p. 12, 1. 6 to 9, p. 13, second paragraph) various sources for the quenching medium: a part-stream 15 branched from the phase separator 9 provided downstream of the quencher 3 (which necessarily also comprises isocyanate liquefied in the quencher), fresh solvent 19, a portion of the liquid phase 13 obtained in the phase separator, and a part-stream of the two-phase product stream 7. The specification does not disclose an embodiment in which the quenching medium is obtained from the gas phase 11 obtained in the phase separator 9 and recycling of isocyanate liquefied in the quencher is dispensed with completely.
In WO 2010/115908 A2, a specific form of the quencher is disclosed. In order to prevent subsequent reactions of the reaction gas at or downstream of the quenching stage, the quenching nozzles and the arrangement thereof are so designed that largely complete wetting of the wall in the quenching region takes place. The entire reaction mixture is thereby included. There are proposed as quenching liquids solvents as well as mixtures with isocyanate or crude mixture from the phosgenation reaction, optionally after particle removal.
EP 2 463 273 A1 discloses a process variant for isocyanate concentrations of greater than 70% by mass in the liquid bottom product leaving the quenching zone. The stream leaving the quenching zone in gas form is passed directly into a jacket-cooled condenser, without passing through a washing column. The gas stream that remains is fed directly to phosgene recovery. Despite the high temperature and high isocyanate concentration in the liquid bottom product of the quenching zone, no information is given regarding residual isocyanate contents in the gas stream that remains. The condensate stream is combined with the condensate of the vapour stream formed by relaxation of the liquid bottom product from the quenching zone, and is fed back as quenching liquid.
The specifications hitherto were concerned, in the quencher region of the gas-phase phosgenation of diamines, substantially with optimising the actual quencher systems. Apart from a few exceptions, the peripheral systems associated with the quencher were disregarded. The exceptions are, for example, as mentioned above, the use of condensation mixture from the liquid collecting vessel of the quencher or of the condensate of the vapour stream formed by flash vaporisation of the condensation mixture, or the use as the quenching liquid of the washing liquid from the gas washing of the vapours leaving the condensate collecting vessel after the quencher. By using condensation mixture from the liquid collecting vessel of the quencher, the amount of solvent required for cooling the reaction gas is reduced by replacing externally supplied quenching liquid by diisocyanate produced in the gas-phase phosgenation and solvent already comprised in the condensate. Likewise, the total amount of quenching liquid can be reduced by using some of the washing liquid from the gas washing of the vapours leaving the condensate collecting vessel after the quencher. Both procedures lead to a reduction of the solvent circulating in the process as a whole and accordingly, when used successfully, contribute towards reducing the energy consumption and can optionally contribute towards reducing costs in terms of apparatus.
A typical process of the prior art will be explained by way of example by means of the first figure (FIG. 1):
The gaseous crude product (101), consisting predominantly of isocyanate, hydrogen chloride and phosgene used in a superstoichiometric amount, is cooled quickly in the quencher (A11) by injection of quenching liquid (105 and 116) in order to avoid undesirable subsequent reactions. The stream (102) that leaves the quencher in the liquid state and comprises especially isocyanate and quenching liquid is passed for the smaller part to product purification and for the larger part via the quench cooler (W11) to the quencher as quenching liquid. Owing to the large circulating stream, a pump (P11) of an appropriately large size must be installed, and thermal stress of the product by repeated contacting with the hot gas stream from the reaction must be taken into account, which leads to losses of yield and an increased outlay in terms of working up. Solids and high boilers formed in or introduced into the reaction or quenching zone pass with the liquid quench product into the quench cooler (W11) and the quenching nozzles and can cause contamination there.
The substance stream (106) that leaves the quencher in gas form and comprises especially vaporised quenching liquid, hydrogen chloride and phosgene is passed to the washing column (A12), in order to remove residual contents of isocyanate from the vapour stream as completely as possible. The higher the content of isocyanates in the substance stream (106) that is supplied, the greater the washing liquid stream (solvent) required at the top of the washing column, and the greater the number of separator stages required for reliable retention. The washing liquid stream is composed of the condensate (113) of the condenser (W12) and additional solvent (110), which may comprise low boilers such as phosgene but is virtually isocyanate-free. The virtually isocyanate-free vapour stream (114) comprises especially phosgene and hydrogen chloride. The liquid bottom run-off (115) comprises especially solvent and is advantageously fed to the quencher as quenching liquid.
The temperature of the substance streams at the outlet of the quenching step must be adjusted while balancing contrary aims, inter alia, on the one hand, between low thermal product stress and low solvent and isocyanate content in the vapour stream to be purified and recycled (low temperature is positive) and, on the other hand, low carbamic acid chloride formation and low energy requirement in the product working-up (high temperature is positive). The temperature of the substance streams at the outlet of the quenching zone is determined, with a given gas stream from the reaction zone, by the amount, temperature and composition of the quenching liquid streams.
On the basis of this prior art, there was a need for a further optimisation of the quenching of a gaseous isocyanate crude product. In particular, the process sections of (a) transfer of the liquid crude product mixture from the quencher into the further working-up stages, (b) treatment of the gas phase (vapours) obtained in the quencher, including gas washing, and (c) supply of the quenching stage against the background of low operating and apparatus costs with high availability and simple, reliable controllability were to be optimised further.